In an article entitled "The C-Patch: A Small Microstrip Element", 15 Dec. 1988, G. Kossiavas, A Papiernik, J. P. Boisset, and M. Sauvan describe a radiating element that operates in the UHF and L-bands. The dimensions of the C-patch are smaller than those of conventional square or circular elements operating at the same frequency, which are relatively bulky. In general, the dimensions of any radiating element are inversely proportional to the resonant frequency. Referring to FIG. 1, a substantially square electrically conductive radiating element or patch 5 (operating at 413 MHz) has an aperture that extends part way across the patch. The width (d) of the aperture (12.5 mm) is shown to be 20% of the total width (L=W=62.5 mm) of the patch, while for an example operating at 1.38 GHz (L-band) the width (d) of the aperture (5.5 mm) is approximately 16.7% of the width (L=22 mm, W=33 mm) of the patch. This antenna geometry is shown to exhibit a threefold to fourfold gain in area with respect to conventional square or circular antennas, although the bandwidth is somewhat narrower. Good impedance matching with a coaxial feed is shown to be a feature of the C-patch antenna, as is an omnidirectional radiation pattern with linear polarization.
In general, microstrip antennas are known for their advantages in terms of light weight, flat profiles, low manufacturing cost, and compatibility with integrated circuits. The most commonly used microstrip antennas are the conventional half-wavelength and quarter-wavelength rectangular patch antennas. Other microstrip antenna configurations have been studied and reported in the literature, such as circular patches, triangular patches, ring microstrip antennas, and the above-mentioned C-patch antennas.
In the "Handbook of Microstrip Antennas", Volume 2, Ch. 19, Ed. by J. R. James and P. S. Hall, P. Peregrinus Ltd., London, U.K. (1989), pgs. 1092-1104, a discussion is made of the use of microstrip antennas for hand-held portable equipment. A window-reactance-loaded microstrip antenna (WMSA) is described at pages 1099 and is illustrated in FIGS. 19.33-19.36. A narrow reactance window or slit is placed on the patch to reduce the patch length as compared to a quarter-wavelength microstrip antenna (QMSA). The value of the reactance component is varied by varying the width (along the long axis) of the slit. FIG. 19.36a shows the use of two collinear narrow slits that form a reactance component in the antenna structure, enabling the length of the radiation patch to be shortened.
The narrow slit does not function as a radiating element, and is thus not equivalent in function to the substantially larger aperture in the above-described C-patch antenna.
So-called PC cards are small form-factor adapters for personal computers, personal communicators, or other electronic devices. As is shown in FIG. 7, a PC card 1 is comparable in size and shape to a conventional credit card, and can be used with a portable computer system 2 that is equipped with an interface 3 that is physically and electrically compatible with a standard promulgated by the Personal Computer Memory Card International Association (PCMCIA). Reference in this regard can be made to Greenup, J. 1992, "PCMCIA 2.0 Contains Support for I/O Cards, Peripheral Expansion", Computer Technology Review, USA, 43-48.
PC cards provide the flexibility of adding features after the base computer system has been purchased. It is possible to install and remove PCMCIA PC cards without powering off the system or opening the covers of the personal computer system unit.
The PC card 1 has standard PCMCIA dimensions of 8.56 cm.times.5.4 cm. The thickness of the PCMCIA card 1 varies as a function of type. A Type II PCMCIA PC card is defined to have a thickness of 0.5 cm. The Type II PCMCIA PC card can be used for memory enhancement and/or I/O features, such as wireless modems, pagers, LANs, and host communications.
Such a PC card can also provide wireless communication capability to laptop, notebook, and palmtop personal computers, and any other computer system having a PCMCIA-compatible interface. The PC card may also work as a standalone wireless communication card when it is not connected to a computer.
For such applications it is required to provide the PC card with a small, built-in antenna having a wide bandwidth isotropic radiation pattern. Since the PCMCIA wireless communication card may be hand-held and/or used in an operator's pocket, the antenna should be substantially immune from effects caused by the close proximity of the human body. Furthermore, the portable PCMCIA communication cards are typically randomly orientated during use and, thus, suffer from multipath reflections and rotation of polarization. Therefore, the antenna should be sensitive to both vertically and horizontally polarized waves. Moreover, the antenna should preferably exhibit the same resonant frequency, input impedance, and radiation patterns when used in free space and when used inside a PCMCIA Type II slot in a conventional portable computer.
It can be appreciated the design of an antenna that meets these various requirements, including a wide bandwidth, presents a significant challenge.
Furthermore, there is a growing interest in developing efficient internal integrated antennas for the class of 900 MHz digital cordless telephones. A high performance built-in antenna is required to have a very small size, a compact structure, a wide bandwidth, a quasi-isotropic radiation pattern, and to exhibit a negligible susceptibility to the proximity of the human body. Furthermore, since portable cordless telephones are normally randomly orientated during use, their antennas must be sensitive to both vertically and horizontally polarized waves. External antennas, such as the whip, sleeve dipole, and helical, are sensitive only to one polarization of the radio waves. As a result, they are not optimized for use with the portable cordless telephones in which antenna orientation is not fixed. Moreover, it has been found that when such external antennas are operated in close proximity to a user of the phone, their radiation patterns change significantly. In addition, a significant portion of the radiated power is attenuated by the user's body.
The microstrip antenna is one of the most preferable types for small portable cordless telephones, especially when a built-in antenna is required. Since the microstrip antenna can be made with a very thin and compact structure, it can easily match various types of portable units. The main problem to be considered when using a microstrip antenna is its narrow bandwidth, which is usually less than 1%, depending on the thickness of the antenna. Most portable digital cordless telephones require the antenna to have an impedance bandwidth of, at least, 3% or 4% at 900 MHz.
Parasitic elements gap-coupled to a rectangular patch antenna have been used for improving the impedance characteristics of the conventional half-wavelength rectangular microstrip antennas. In such a case, the parasitic and the driven elements, resonating at adjacent frequencies, give flat impedance characteristics over a wide band of frequencies. However, these configurations increase the overall size of the antenna considerably.